1. Field of the Invention
The invention relates to a radar-based method of measuring the level of a material in a container in which, by means of the antenna of a ranging device arranged above the highest level anticipated, microwaves are radiated downwards and reflected microwaves received, the echo profile representing the amplitude values of the received microwaves as a function of the transit time or the distance corresponding to the transit time is registered, a maximum of the echo profile is determined for detecting the echo blip in the echo profile originating from the echo waves reflected by the material surface and the distance of the material surface from the antenna is obtained from the peak value of this echo blip.
2. Description of the Prior Art
In prior art methods of this type it is assumed that the maximum of the echo profile corresponds to the peak value of the echo blip originating from the echo waves reflected by the material surface so that the distance of the material surface from the antenna can be determined from the distance value assigned to this peak value. It may, however, occur that this requirement is not satisfied when a double echo arrives at the antenna caused by the microwaves being reflected not only by the material surface but also by an interface located just below the material surface. One such interface is formed, for example, by the bottom of the container when the level of material therein is very low. If the material has a small dielectric constant only a relatively minor proportion of the impinging microwaves is reflected by the material surface whilst the remainder of the impinging microwaves passes through the material and is reflected by the bottom of the container. In the case of a low material level and a small dielectric constant the microwaves passing through the material are attenuated to only a minor degree; accordingly it may occur that the echo wave reflected by the material surface has a smaller amplitude than that of the echo wave reflected by the bottom of the container. The two echo waves arrive at the antenna, the echo wave reflected by the bottom of the container being delayed relative to the echo wave reflected by the material surface by an interval in time corresponding to the distance between the material surface and the bottom of the container. When this distance is small, the blips originating from the two echo waves in the echo profile overlap, forming a double blip. Measuring the level on the basis of the maximum of the echo function may then result in a false reading when the echo wave reflected by the bottom of the container has a greater amplitude than that of the echo wave reflected by the material surface due to this maximum not being located at the distance value of the peak value of the echo blip originating from the echo wave reflected by the material surface.
The same phenomenon occurs when the reflecting interface is not the bottom of the container but an interface between two substances having greatly differing dielectric constants located just below the material surface, this applying, for example, to a film of oil floating on water.